The present invention relates to nonlinear resistive devices, such as varistors, and more particularly to methods of making such devices using various plating techniques in which only the electrically contactable end terminals of the device are plated.
Nonlinear resistive devices are known in the art, and are described, for example, in U.S. Pat. No. 5,115,221 issued to Cowman on May 19, 1992, that is incorporated by reference.
With reference to FIG. 1, a typical device 10 may include plural layers 12 of semiconductor material with electrically conductive electrodes 14 between adjacent layers. A portion of each electrode 14 is exposed in a terminal region 16 so that electrical contact may be made therewith. The electrodes 14 may be exposed at one or both of opposing terminal regions, and typically the electrodes are exposed at alternating terminal regions 16 as illustrated. The exposed portions of the electrodes 14 are contacted by electrically conductive end terminals 18 that cover the terminal regions 16.
While an apparently simple structure, the manufacture of such devices has proved complex. For example, the attachment of the end terminals 18 has proved to be a difficult problem in search of a simplified solution. Desirably, the terminal regions 16 may be plated with nickel and tin-lead metals to increase solderability and decrease solder leaching. The process parameters in plating nickel to zinc oxide semiconductor bodies has proved particularly vexing and has required complex solutions.
One method of affixing the end terminals 18 is to use a conventional barrel plating method in which the entire device is immersed in a plating solution. However, the stacked layers are semiconductor material, such as zinc oxide, that may be conductive during the plating process so that the plating adheres to the entire surface of the device. Thus, in order to provide separate end terminals as shown in FIG. 1, a portion of the plating must be mechanically removed after immersion, or covered before immersion with a temporary plating resist comprised of an organic substance insoluble to the plating solution. However, the removal of the plating or organic plating resist is an extra step in the manufacturing process, and may involve the use of toxic materials that further complicate the manufacturing process.
It has also been suggested that the metal forming the end terminals 18 be flame sprayed onto the device, with the other portions of the surface of the device being masked. Flame spraying is not suitable for many manufacturing processes because it is slow and includes the creation of a special mask, with the additional steps attendant therewith. See, for example, U.S. Pat. No. 4,316,171 issued to Miyabayashi, et al. on Feb. 16, 1982.
It is also known to react a semiconductor body, having electrically conductive metal end terminations, with phosphoric acid to selectively form a phosphate on the semiconductor body prior to providing end terminations using conventional barrel plating. However, in this method the phosphate layer is formed by the reaction of the phosphoric acid with the metal oxide at the surface of the body to form an electrically insulative metal phosphate layer. The process stops once the surface of the exposed body has been reacted resulting in a thin phosphate layer which is susceptible to erosion during the plating process. See, U.S. Pat. No. 5,614,074 issued to Ravindranathan on Mar. 25, 1997 and owned by the assignee of the present invention.
Accordingly, it is an object of the present invention to provide a novel method and device that obviates the problems of the prior art.
It is another object of the present invention to provide a novel method and device in which an electrically insulating, inorganic layer is formed on portions of the device before the device is plated.
It is still another object of the present invention to provide a novel method and device in which a phosphoric acid solution is reacted with the exposed surface of stacked zinc oxide semiconductor layers to form a zinc phosphate coating.
It is still another object of the present invention to provide a novel method and device in which a passivation solution reacts with the exposed ceramic surface of the device to form a zinc phosphate coating.
It is still a further object of the present invention to provide a novel method and device in which a semiconductor device is saturated in a phosphoric acid solution to form a zinc phosphate layer by deposition and by reaction of the solution with the device surface.
It is yet another object of the present invention to provide a novel method and device in which a zinc phosphate coating protects portions of the device that are not to be plated when the end terminals are formed.
It is a further object of the present invention to provide a novel method of providing a semiconductor device with an inorganic electrically insulative layer in which a device with exposed semiconductor surfaces and metal end terminations is submerged in phosphoric acid to form a phosphate on the exposed surfaces of the semiconductor, and in which the device is thereafter barrel plated and the plating is provided only on the end terminations because the phosphate is not electrically conductive.
It is yet a further object of the present invention to provide a novel method and nonlinear resistive device having a body of layers of semiconductor material with an electrode between adjacent layers, in which the body of the nonlinear resistive device is coated with an inorganic layer that is electrically insulating, except at a terminal region of the body where an electrode is exposed for connection to an end terminal, and in which the coated body is plated with an electrically conductive metal to form the end terminal in a process in which the body becomes electrically conductive and in which the electrically conductive metal does not plate the coated portions of the body because the inorganic layer is not electrically conductive.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.